Rodimus,
Unless your friend begins acting like he is in charge, then this company will continue to take what they can.
Keeping short accounts in business is like disciplining children. You need short lead times between action and consequence and consistent responses. and the sooner you start, the easier it is to make it work.
So, basically this person needs to say I will not release any work to you and will not continue to do work until you pay me. If he is accruing other costs, then this company will need to pay for them up front. Find the point at which this company does need him and bargain at that point.
A big client who doesn't pay is no use to you. You are far better off with a little client who pays what they owe you! Your friend might offend them, but at the moment, they are taking them for granted and they'd be better looking for an alternative client anyway. The situation they are in is worse than being an slave! They are basically working for nothing - and paying others to work for them. They are on a sure course to bankruptcy if they continue. And this big company can probably smell the fear on them and just keeps on not paying them - he's already shown he's not going to do anything to them.
Stop doing the work for them and begin sending letters of demand and if that doesn't work, start legal action immediately. Once they learn that they can't take them for granted, then possibly consider work for them again - but on your terms not theirs. Being polite and clever doesn't very often work in this situation. You need to stick to your guns and be stronger than they are.
Is he confident that this company he's working for is liquid? I had this situation with a company I used to deal with... and thought it was just me. Then I discovered they weren't paying anyone. It sounds like this company could be the same. Appears very strong, but actually has no money.
Honestly, I would tell your friend to stop working for them as soon as possible, particularly if he is accruing other costs on the way. It's no way to run a business, to be totally dependent upon one person, who is not a consistent payer. Get out of there, fast!
Pen